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Printing ink, like any ink, is a mixture of pigment and carrier. Its manufacture depends on what kind of printing process it will be used in, and any regulations that relate to its composition and performance. The original pigments came from ground lamp black (basically soot) and various animal and vegetable compounds gathered to create a palette of colour for printers. Today's pigments are more likely to be dyes (salts of nitrogen compounds) or chemical compounds, although carbon-black is still the favourite for black pigment, whether in ink or paint. Printers' ink, like writing ink, must cover completely and evenly and dry quickly. Printers generally don't have the luxury of letting pages sit out to dry, although that's just what many artists do when they create lithographs. The printers who run thousands of newspapers several times a day on huge offset presses need an ink that soaks into the paper so that the paper can be cut, folded and distributed while the ink is still drying. That's why newsprint comes off on your hands when you handle the latest edition.
The major difference between classic ink recipes and modern printers' ink, is in the carrier used to apply the pigment to paper. Most classic recipes used linseed oil, a natural substance that grows thicker as it is heated until it becomes varnish. Other resins were used but linseed oil was the favourite because of its ability to dry quickly. Other than the introduction of new resins (alkyds) and oils (mineral, soybean), the process has remained unchanged for the past few hundred years. The carrier is heated to between 93.3 to 315 degrees C and cooked for as much as 12 hours to achieve the proper thickness. Letterpress and lithographic inks are "oil" or "paste" inks and the carrier cooks longer. Thinning solvents are added to the resulting mixture to create carriers for flexographic and rotogravure. A different mixture, with a dryer, is required for sheet-fed offset, used for colour printing. Heat-set web offset uses a composition that responds to heat so it dries more quickly.
While the carrier cooks, the pigment is prepared by grinding and drying in a roller mill (the same technology used in grain mills of the nineteenth century) until it is microscopically fine. The carrier and pigment are then mixed and additives such as wax, surfactants, lubricants and drying agents, depending on the print process for which the ink will be used. The whole idea is to make the ink thick enough to cover and thin enough to dry easily; to transfer easily from the press to the paper (whether an early lithographer's stone or a modern offset press) with a minimum of wasted ink and resultant clean-up. Current developments in the industry centre on the use of soybean and other vegetable oils, Volatile Organic Compound (VOC) reduction using ultraviolet drying techniques, water-based dyes and other sustainable materials.
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